This invention relates to microporous membranes and processes for making such membranes. In particular, it relates to polytetramethylene adipamide or nylon 46 microporous membranes having narrow pore-size distributions. Such membranes are, therefore, useful, for example, for efficient filtration of particulates from liquids, and especially from aqueous liquids. They are also useful as transfer media.
A microporous membrane is a porous solid which contains microporous interconnecting passages that extend from one surface to the other. These passages provide tortuous tunnels through which the liquid which is being filtered must pass. The particles contained in the liquid passing through a microporous membrane become trapped on or in the membrane structure effecting filtration. A slight pressure, generally in the range of about 5 to 50 psig (pounds per square inch gauge) is used to force fluid through the microporous membrane. The particles in the liquid that are larger than the pores are either prevented from entering the membrane or are trapped within the membrane pores. The liquid and particles smaller than the pores of the membrane pass through. Thus, a microporous membrane prevents particles of a certain size from passing through it, while at the same time permitting liquid and particles smaller than that size to pass through. The microporous filter membranes have the ability to retain particles in the size range from about 0.1 to about 10.0 microns.
Many important micron and submicron size particles can be separated using microporous membranes. For example, red blood cells are 8 microns in diameter, platelets are about 2 microns and bacteria and yeasts may be 0.5 microns or smaller. It is possible to remove bacteria from water by passing the water through a microporous membrane having a pore size smaller than the bacteria. Similarly, a microporous membrane can remove invisible suspended particles from water used in the manufacture of integrated circuits in the electronics industry. Microporous membranes are characterized by bubble point tests, which involve measuring the pressure to force either the first air bubble out of the membrane (initial bubble point) or force air bubbles out all over the membrane (foam-all-over-point or "FAOP"). The procedures for conducting initial bubble point and FAOP tests are well known in the art. The procedures for these tests are explained in detail for example in ASTM F316-70 and ANS/ASTM F316-70 (Reapproved 1976) which are incorporated herein by reference. The bubble point values for microporous membranes are generally in the range of about 5 to about 100 psig, depending on the pore size.
Microporous membranes are distinguishable from semipermeable membranes which include ultrafiltration (U.F.) and reverse osmosis (R.O.) membranes. Ultrafiltration membranes are used for molecular separation rather than particle filtration, i.e., they are used to separate out large molecules, including proteins and dye molecules in the size range of 0.001 micron (10 Angstroms) to 0.1 micron (1000 Angstroms). Unlike ultrafiltration and microporous membranes, the reverse osmosis (R.O.) membranes do not act as sieves. Instead of retaining larger molecules, the reverse osmosis (R.O.) membranes allow certain selected molecules to pass through them. The passage of these molecules is determined by the chemical affinity of the molecules toward the membrane material. The molecule which passes through the membrane may or may not be smaller than those retained.
Strictly speaking, the reverse osmosis membranes are not filtration membranes. However, the terms "molecular filtration" or "hyperfiltration" are sometimes used to describe the operation of these membranes.
The semipermeable membranes, i.e., ultrafiltration and reverse osmosis membranes, possess a thin non-porous outer layer that is usually supported on a much thicker support layer. The outer layer is sometimes referred to as a "skin." The "skin" of a semi-permeable membrane is non-porous in the micron range; however, it does contain molecular size openings which are measured in Angstroms. One Angstrom is one ten thousandth (1/10,000) of a micron. The separation in semi-permeable membranes is controlled by the skin and the skin must be thin to reduce the overall resistance to flow. To maintain the integrity of the reverse osmosis (R.O.) membrane, a much thicker porous layer is present. Semi-permeable membranes separate molecules. Accordingly, to achieve separation using semipermeable membranes, significantly higher pressures than those for microporous membranes are required. For example, the semipermeable R.O. membranes of U.S. Pat. No. 3,703,570 (Busch) required about 600 psig pressure to achieve 50% salt rejection. Likewise, pressure of about 600 psig was required to achieve 95% separation of a salt from a salt solution using the semipermeable membranes disclosed in U.S. Pat. No. 3,699,038 (Boom). The bubble point tests are not applicable or used for the characterization of semipermeable membranes. Instead, such membranes are characterized by measurement of salt or other solute rejection.
Similarly, microporous membranes should be distinguished from artificial leather materials, such as those described in the Japanese patent application 26749-1969 (Teijin), U.S. Pat. No. 3,208,875 (Holden) U.S. Pat. No. 3,000,757 (Johnston) and U.S. Pat. No. 3,190,765 (Yuan). The Japanese patent application of Teijin discloses a process for making a leather substitute that is composed of unconnected cells similar to those present in urethane foam. When reproduced, Examples 3 and 4 of the Teiun Japanese patent application, produced foam-like nylon materials. The leather materials of Examples 3 and 4, made of nylon that is alcohol insoluble at room temperature, had skins on both surfaces of the sheet.
Similarly, the water vapor permeable sheet materials of the U.S. Pat. No. 3,208,875 (Holden), U.S. Pat. No. 3,190,765 (Yuan) and U.S. Pat. No. 3,000,757 (Johnston) patents are examples of other patents disclosing polymeric sheet materials useful as leather substitute materials. Like Teijin materials, these sheets do not contain micropores formed of interconnected passages providing tortuous tunnels from one side of the material to the other, characteristic of microporous membranes. In addition, these leather-substitute materials contain thin non-porous skins on one or both of their surfaces. Because of these skins and the essentially unconnected cells, the artificial leather materials do not permit the flow of liquid through them. Instead, they allow vapors to diffuse through the material between the cells and through the skin. Accordingly, the artificial leather materials are generally characterized by a leather permeability value (LPV), 5000 grams per hour per 100 square meters determined by the test described by Kanagy and Vickers in the Journal of the American Leather Chemists Association 45, 211-242 (Apr. 19, 1950).
The original method for making nylon microporous membranes for filtration was a slow and inefficient vapor equilibration process, sometimes referred to as "the dry" process. A dry process for making alcohol-soluble nylon membranes is described in U.S. Pat. No. 3,408,315 (Paine). In that process, the liquid components of the polymeric solution are selected based on different relative volatilities. A solution is prepared from a polymer, a more volatile (easily evaporated) solvent and a less volatile (less easily evaporated) component. The process is based on the principle that the more volatile component evaporates faster causing a gradual change of concentration eventually leading to precipitation of the polymer.
The polymer solution is spread onto a surface, and then subjected to a slow and cumbersome controlled multi-stage evaporation in vapor equilibration chambers. Each successive stage contains slightly less concentrated vapor of the volatile component of the solution, allowing increasingly more solvent to evaporate from the spread nylon solution until an equilibrium is reached in that stage. The more volatile solvent is gradually removed by evaporation until the polymer concentration in the solvent becomes high enough for the polymer to precipitate forming a microporous structure. In order to obtain a membrane with micropores throughout, i.e., without a thin skin, the evaporation of the solvent has to be carried out at each stage near equilibrium conditions. Otherwise, the solvent would evaporate preferentially from the surface causing a high local concentration of the polymer and locally precipitating the polymer there. The remainder of the membrane would not be formed till later. A rapid evaporation thus causes a thin semipermeable or an entirely solid skin. Accordingly, the rate of the dry process is limited by the need to slowly evaporate the more volatile component in order to maintain vapor equilibrium. After the membrane is formed in the equilibrium chambers, the solvents are washed off and the resulting microporous membrane is dried. Millipore Company manufactured and sold alcohol-soluble hydrophilic microporous nylon membranes under the trademark DURALON from about 1964 to about 1975.
U.S. Pat. No. 3,876,738 (Marinaccio-Knight) discloses the first rapid and efficient process to make a nylon microporous membrane. A nylon dope solution is prepared and immersed into a quench bath containing a non-solvent system, without the slow and cumbersome equilibration step of the dry process. Marinaccio and Knight were first to discover that direct immersion (the "wet process") could be used to make nylon microporous membranes catastrophically, preferably from alcohol-insoluble nylon such as, nylon 6, 66 or 610, using formic acid as the solvent.
The Marinaccio-Knight patent discloses that by altering the characteristics of the nylon solvent in a particular way the formation of a thin skin or a cellular structure can be avoided. A membrane with micropores throughout could thereby be formed catastrophically, as long as the modified nylon solution is solidified (cast into a membrane) entirely under the surface of the quench liquid.
The Marinaccio-Knight patent also discloses that the solvent must be altered to make an aggregated polymer solution in order to make nylon membranes which are microporous throughout. Such a solution could be achieved by modifying formic acid (a good solvent) with a non-solvent (a dopant) that has a different solubility parameter, such as, water, methanol glycerin, and/or methyl formate. The modified (doped) solubility parameter of the solvent system is achieved by adding a non-solvent that has a different solubility parameter from that of the solvent.
The solubility parameter of a formic acid-methanol-water solvent system are a function of composition. The amount of the nonsolvent and the type of non-solvent must be such as to modify the solubility parameter of the formic acid to induce the proper aggregation of the nylon.
The Marinaccio-Knight patent further discloses that a non-solvent can be selected for use in the quench bath based on mutual miscibility (solubility) with the solvent and when present, the non-solvent used in the dope solution. Where the same non-solvent and solvent are used in the quench bath as in the dope solution, the ratio of solvent to non-solvent should be lower in the quench bath so that casting of the microporous microstructure occurs beneath the surface of the quench bath of catastrophic precipitation of the dope solution into a solid structure. The patent discloses a range of suitable quench bath formulations.
The Marinaccio-Knight patent discloses that the solvent system employed in the polymer solution is one of the key parameters "responsible for the development of micropores in the film." Column 2, lines 37-38. Unlike a non-solvent system which may comprise only a non-solvent, the solvent system of the Marinaccio-Knight patent includes a combination of materials. Column 2, lines 6-10. The nature of the solvent system can be empirically determined on the basis of solubility parameters. Column 2, lines 4-61. The solubility parameter of a solvent system can be changed by the addition of a third component. Column 2, line 67--Column 3, line 4. The solubility parameter that governs aggregation of the molecules is the solubility parameter of the mixture and the patent discloses how to calculate it for a given mixture. Column 3, line 63 through Column 4, line 4. The patent further discloses that the proper aggregation of the polymer to make microporous membranes can be achieved by addition of a non-solvent or other additives. Column 4, lines 33-36. For a specific application of the general teaching, the patent then refers one skilled in the art to the procedures set forth in the Examples. Column 4, lines 41-47.
The Marinaccio-Knight patent identifies the important process parameters: polymer, solvent system, quench bath composition, polymer composition, age of polymer solution, time of quench, quench and solvent temperature and quench bath additives. Column 2, line 19-25. This patent further discloses that the "preferred" film forming polymers are nylon polymers, especially alcohol-insoluble nylon polymers. Column 5, lines 52-53. The patent specifically identifies three alcohol-insoluble nylons: nylon 6 (Allied A 8205), 610 (Zytel-31) and 66 (condensation product of hexametylenediamine and adipic acid). Column 7, line 19, column 8, line 26, and column 9, lines 16-17.
The patent also discloses that the film structure is formed catastrophically; i.e., without the slow equilibration step of gelling in a controlled atmosphere. However, the dope solution can be exposed to an atmosphere saturated in nylon solvent system, i.e., an atmosphere rich in formic acid and water vapors, prior to being solidified in the quench bath. Column 6, lines 51-61.
Next, the Marinaccio-Knight patent discloses the application of the invention to making specific microporous alcohol-insoluble nylon, nylon 6 made by Allied Corporation and sold under the trade name A-8205. Thus, Example 1 gives a description of the procedure for making nylon microporous membrane having a pore size of 1.00 micrometer (micron). Example 1 represents the most convenient method for formulation of the dope solution. However, as one skilled in the art would immediately recognize, methanol and formic acid react. Therefore, Example 2 is provided to show the dope solution having the same ingredients as Example 1 once ultimate chemical equilibrium is achieved.
Example 3 teaches one skilled in the art that increasing the proportion of water in the quench bath from 50% to 70% reduces the pore size of the resulting membrane.
Example 4 teaches one skilled in the art that using about a 50% smaller amount of glycerol than methanol produces membrane which has about a 20% larger pore size. In other words, glycerol dopes the formic acid solubility parameter in a different way, reflecting its different solubility parameter.
Example 5 teaches one skilled in the art that a slight lowering of methanol level and concurrent increasing of the water to 70% in the quench bath changes the pore size.
Example 7 teaches that other nylon 6 and Zytel 31--nylon 610--produce similar results.
Example 6 is a factorial experiment which defines the outer working limits for the nylon 6, formic acid and methanol system of Example 1. The experiment varies the three principal process variables for nylon 6: (1) nylon polymer concentration, (2) non-solvent concentration in the solvent system and (3) the composition of the quench bath. Example 1, the preferred composition, is identified at the center of the cube. Given the outer limits, one skilled in the art can select the process variables to produce a desired nylon microporous membrane.
U.S. Pat. No. 4,340,479 (Pall) is directed toward hydrophilic nylon microporous membranes from nylon resins which have the ratio of methylene groups to groups of about 5:1 to 7:1, to the process for making such membranes, and to products incorporating such membranes. Briefly, the Pall patent discloses nylon membrane material which is said to be unique in that it is composed of a nylon resin which in its bulk form is hydrophobic but which is transformed into a hydrophilic membrane material. The membrane material is further asserted to be distinguishable from other nylon membrane materials in that it "reverts," when heated to a temperature just below the softening temperature of the membrane. Just below this softening temperature, the material of the Pall patent membrane is asserted to become so hydrophobic that is no longer wetted by water.
Pall asserted that the step of the process which transforms the hydrophobic resin into a hydrophilic microporous nylon membranes is the step of "nucleation of the casting solution." This "nucleation" is achieved by a controlled addition (including the rate of addition) of a nonsolvent to the nylon polymer-solvent solution so that a visible precipitate is formed. For example, the Pall patent states that:
In accordance with the invention, alcohol-insoluble polyamide resin membrane sheet is provided that is inherently hydrophilic. This is a most remarkable property, inasmuch as the alcohol-insoluble polyamide resin from which the sheet is made is hydrophobic. PA1 The phenomenon of hydrophilicity arises primarily as the result of nucleation of the casting resin solution. PA1 In accordance with the invention, alcohol-insoluble polyamide resin membrane sheet is provided that is inherently hydrophilic. This is a most remarkable property, inasmuch as the alcohol-insoluble polyamide resin from which the sheet is made is hydrophobic. The phenomenon occurs only with alcohol-insoluble polyamide resins having a ratio CH.sub.2 :NHCO of methylene CH.sub.2 to amide NHCO groups within the range from about 5:1 to about 7:1. PA1 Skinned membranes behave very differently; when water wetted and their air flow-pressure drop relationship is determined, the curve is not flat initially, but slopes upward, indicating presence of large pores; transition to a more nearly vertical line is slow, with a large radius, and in the "vertical" area, instead of the sharp rise of FIG. 3, a sloping line is obtained, reflecting a wide pore size range. Such membranes are poorly suited to obtain sterile filtrates when challenged by bacteria; either a nonsterile fluid is obtained, or if sterility is gotten, it is at the cost of very high pressure drop to achieve a low throughput rate.
In his Declaration, Dr. Pall attributed the creation of hydrophilicity in the hydrophobic starting resin to the step of "nucleation:"
The Pall patent teaches that hydrophilic microporous (i.e., skinless) membranes can be produced by the process of that invention only from polyamide resins having CH.sub.2 :NHCO (methylene to amide groups) within the range from about 5:1 to about 7:1.
Column 8, lines 18-26. See also column 9, lines 16-25.
Pall disclosed nylons 6, 66 and 610 as being preferred, the same nylons specified in the Marinaccio-Knight patent.
The Pall patent refers to its microporous membranes as being "skinless." The patent defines "skinless" microporous membranes as those not having a K.sub.L profile of "skinned" membranes:
Column 26, lines 33-44.
In summary, prior to this invention the production of nylon from alcohol-soluble nylons by the "dry" (equilibrium) process was known. The Marinaccio-Knight process disclosed a process for production of nylon membranes from both alcohol soluble and alcohol-insoluble nylons by a rapid immersion process in which the structure of the membrane was formed under the surface of the quench bath. The Pall patent taught that hydrophilic skinless nylon membranes were made only from hydrophobic polyamide resins which have the ratio of methylene to amide groups within the range from about 5:1 to about 7:1.
U.S. Pat. No. 4,788,226 (Curry) discloses skinless polyamide hydrophilic membranes suitable for use in microfiltration. These membranes are made of polytetramethylene adipamide (Nylon 46), either alone, or in admixture with at least one other polyamide. The Curry patent discloses that the additional polyamides can include polyhexamethylene adipamide (Nylon 6, 6), poly-e-caprolactam (Nylon 6) and polyhexamethylene sebacamide (Nylon 610). It also discloses that the Nylon 46 membranes could be cast using either a wet process or a dry process. Column 2, lines 27-29.
The present invention goes against the teachings of the prior art and specifically of the Pall patent and provides a hydrophilic "skinless" (under the K.sub.L test definition "K.sub.L test skinless") microporous membrane from a nylon resin which is outside the range of ratios of methylene to amide groups specified in the Pall patent. Moreover, contrary to the assertions made in the Pall patent, membranes are made without the need for forming a visible precipitate in making a dope solution. Finally, contrary to the teachings of the Pall patent, membranes which are "skinless" under Pall's definition of this term are produced in a quench bath which includes only small amounts of formic acid.
The present invention provides membranes of polytetramethylene adipamide which have narrow pore-size distributions and a process for making such membranes. The present invention also provides membranes which have pore-size distribution so narrow that they are useful as transfer media in transfer of macromolecules from a chromatographic substrate to an immobilizing matrix.